104 INSECT LIFE. 



The Surface-film of Water {School Work). — 

 One of the astonishing facts to be learned by the 

 study of pond life is that many insects are able to 

 walk upon water, and equally strange is the fact that 

 certain other insects that live within water can hang 

 from its upper surface without any effort to keep 

 themselves there, although their bodies are heavier 

 than water. 



These things seem to contradict the well-known 

 law of Nature that an object can not float in a liquid 

 unless it is lighter than that liquid. And in truth 

 it is a fact that under certain conditions objects that 

 are much heavier than the liquid upon which they 

 are placed will float. 



This phenomenon is due to the presence of what 

 has been termed the surface-film of liquids. It has 

 been shown by many experiments, some of which 

 are given below, that on the surface of water, and 

 other liquids also, there is a film of the liquid which 

 tends to contract as if it were a membrane stretched 

 equally in all directions. The explanation of the for- 

 mation of this film is a difficult matter, which can not 

 be undertaken here. It is discussed in the more ad- 

 vanced works on physics and in some of the larger 

 cyclopaedias under the head of capillary attraction. 

 The action of the surface-film can be seen by the 

 following experiments : 



Experiment i. — Take a fine needle and carefully 

 lay it on the surface of a glass of water. To do this 

 hold the needle in a horizontal position and bring it 

 as near the water as you can without touching the 

 water and then drop the needle. If the experiment 

 be performed with sufficient care, the needle will 



